Liquid crystals have found use in a variety of electro-optical and display device applications, in particular those which require compact, energy-efficient, voltage-controlled light valves such as watch and calculator displays. These devices are based upon the dielectric alignment effects in nematic, cholesteric and smectic phases of the liquid crystal compound in which, by virtue of dielectric anisotropy, the average molecular long axis of the compound takes up a preferred orientation in an applied electric field. Since the coupling to an applied electric field by this mechanism is rather weak, the resultant electro-optical response time may be too slow for many potential applications.
Liquid crystal displays have a number of unique characteristics, including low voltage and low power of operation, which makes them perhaps the most promising of the nonemissive electro-optical display candidates available with today's technology. However, slow response and insufficient non-linearity can impose limitations for many potential applications. The requirement for speed may become especially important in proportion to the number of elements which have to be addressed in a device. This may result in increasingly impractical production costs for the potential use of such devices in flat-panel displays for use in video terminals, oscilloscopes, radar and television screens.
It has been shown by N.A. Clark and S.T. Lagewall in Appl. Phys. Lett. 36:899 (1980) and in U.S. Pat. No. 4,367,924 that electro-optic effects with sub-microsecond switching speeds are achievable using the technology of ferroelectric liquid crystals. Some ferroelectric liquid crystal display structures, in addition to the high speed (about one thousand times faster than currently used twisted nematic devices) reported by these investigators, exhibit bistable, threshold sensitive switching, making them potential candidates for matrix addressed light valves containing a large number of elements for passive displays of graphic and pictorial information, as well as for optical processing applications.
A basic requirement for application of ferroelectric liquid crystals in such devices is the availability of chemically stable liquid crystal compounds which exhibit ferroelectric phases over a substantial temperature range about room temperature. Ideally, these compounds, which must be chirally asymmetric to be ferroelectric, would exhibit a large ferroelectric dipole density, (P) in order to optimize coupling to an applied electric field, a low orientational viscosity (.eta.) in order to optimize response times, and chemical and photochemical stability.
U.S. Pat. No. 4,556,727 discloses chirally asymmetric liquid crystal compounds formed by the incorporation of enantiomerically enriched tail units derived from readily available and inexpensive ethyl lactate into a liquid crystal molecular framework. More specifically, that patent discloses that the attachment of an enantiomerically enriched lactic acid derived tail unit to the para position of the phenyl group of a phenyl benzoate core unit will confer the desired properties of large ferroelectric dipole density and low orientational viscosity to a chirally asymmetric liquid crystal compound. The disclosure of that patent relates to ferroelectric smectic liquid crystals of the following general formula: ##STR2## wherein R is a lower alkyl group containing one to three carbon atoms.
U.S. patent application of Walba and Vohra, Ser. No. 782,348, filed Oct. 1, 1985, now U.S. Pat. No. 4,638,073, discloses ferroelectric (chiral) smectic liquid crystal compounds having an achiral core and chiral tail units derived from (2,3)-epoxyalkyloxiranemethanols which possess a high ferroelectric polarization density. The ferroelectric crystals have the following general formulas: ##STR3##
U.S. application of Walba and Eidman, Ser. No. 880,851, filed July 1, 1986, discloses chirally asymmetric liquid crystals possessing the phenyl benzoate core unit and 1-cyanoalkoxy chiral tails.
While useful liquid crystal materials have thus been reported, optimum response times have not been achieved. This is partly because of the relatively low ferroelectric polarization densities of many known materials.